  | |
| Observation data EpochJ2000.0 EquinoxJ2000.0 (ICRS) | |
|---|---|
| Constellation | Octans |
| Right ascension | 21h 08m 46.86357s[1] |
| Declination | −88° 57′ 23.3983″[1] |
| Apparent magnitude (V) | 5.47[2] |
| Characteristics | |
| Spectral type | F0 IV[3] |
| U−Bcolor index | +0.13[2] |
| B−Vcolor index | +0.26[2] |
| Variable type | δ Sct[4] |
| Astrometry | |
| Radial velocity (Rv) | +11.9[5] km/s |
| Proper motion (μ) | RA: +26.323[1]mas/yr Dec.: +4.721[1]mas/yr |
| Parallax (π) | 11.1005±0.0616 mas[1] |
| Distance | 294 ± 2 ly (90.1 ± 0.5 pc) |
| Absolute magnitude (MV) | 0.86±0.09[6] |
| Details | |
| Mass | 1.59[7] M☉ |
| Radius | 4.4[1] R☉ |
| Luminosity | 44[1] L☉ |
| Surface gravity (log g) | 3.71[7] cgs |
| Temperature | 7,415±252[7] K |
| Metallicity [Fe/H] | −0.5[1] dex |
| Rotational velocity (v sin i) | 145[4] km/s |
| Age | 912[7] Myr |
| Other designations | |
| Polaris Australis,σ Oct,CPD−89°47,FK5 923,HD 177482,HIP 104382,HR 7228,SAO 258857[8] | |
| Database references | |
| SIMBAD | data |
Sigma Octantis is a solitary[9]star in theOctansconstellation that forms thepole star of theSouthern Hemisphere. Its name is also written asσ Octantis, abbreviated asSigma Oct orσ Oct, and it is officially namedPolaris Australis (/poʊˈlɛərɪsɔːˈstreɪlɪs/).[10] The star is positioned one degree away from the southerncelestial pole of theSouthern Hemisphere, lying in nearly opposite direction to theNorth Star on thecelestial sphere.
Located approximately294 light-years from Earth, it is classified as asubgiant with aspectral type of F0 IV. Sigma Octantis has anapparent magnitude of 5.5, but is slightly variable and is classified as aDelta Scuti variable.
σ Octantis (Latinised toSigma Octantis) is the star'sBayer designation.
As the southern hemisphere's pole star it bore the namePolaris Australis, first applied in the 1700s.[12] In 2016, the IAU organized aWorking Group on Star Names (WGSN)[13] to catalog and standardize proper names for stars. The WGSN approved the namePolaris Australis for this star on 5 September 2017 and it is now so included in the List of IAU-approved Star Names.[10] It is the southernmost named star.
With a spectral class of F0IV,[3] Sigma Octantis appears to be asubgiant, although it has also been classified as F0III.[14]Evolutionary models place it at the very end of itsmain sequence life with an age of about 900 million years.[1][7] It has expanded somewhat to a size 4.4that of the Sun and emits 44 times as muchelectromagnetic radiation from itsphotosphere at aneffective temperature of7,415 K.
Sigma Octantis is aDelta Scuti variable, varying by about 0.03 magnitudes every 2.33 hours.[4] It is thought to pulsate only in thefundamental mode.[15]
Sigma Octantis is the current southernpole star, whose counterpart isPolaris, the current North Star. To an observer in the southern hemisphere, Sigma Octantis appears almost motionless and all the other stars in the Southern sky appear to rotate around it. It is part of a small "half hexagon" shape. It is slightly more than a degree away from the true south pole, and the south celestial pole is moving away from it due toprecession of the equinoxes.
At magnitude +5.42, Sigma Octantis is barely visible to the naked eye, making it unusable for navigation, especially by comparison with the much brighter and more easily visiblePolaris.[16]Because of this, theconstellationCrux is often preferred for determining the position of theSouth Celestial Pole.[17]Once Sigma Octantis' approximate position has been determined, either by the major stars inOctans or using the Southern Cross (Crux) method, it can be positively verified using anasterism: Sigma,Chi,Tau, andUpsilon Octantis are all stars of around magnitude 5.6, and form the distinctive shape of atrapezoid.
Sigma Octantis was used as a reference to measure the magnitudes of stars in the southern hemisphere for the1908 Revised Harvard Photometry catalogue.The Pole Star andLambda Ursae Minoris were used for the northern hemisphere. It was then noted that "Neither of these stars appears to vary perceptibly" but that, due to the procedures used "if they did, the variation would have no effect on the final measures."[18]
Sigma Octantis is the dimmest star to be represented on a national flag. It appears on theflag of Brazil, symbolising theBrazilian Federal District.[19]
